In general, the sintered ore as a main raw material for a blast furnace iron-making method is manufactured through a process as shown in FIG. 1. The raw material for the sintered ore includes iron ore powder, undersize granules of the sintered ore, recovery powder produced in an ironworks, a CaO-containing auxiliary material such as limestone, dolomite or the like, a granulation auxiliary agent such as quicklime or the like, coke breeze, anthracite and so on, which are cut out from respective hoppers 1 onto a conveyer at a predetermined ratio. The cut-out raw material is added with a proper amount of water, mixed and granulated in drum mixers 2 and 3 to form quasi-particles having a mean particle size of 3˜6 mm as a sintering raw material. Then, the sintering raw material is charged onto a pallet 8 of an endless-moving type sintering machine at a thickness of 400˜800 mm from surge hoppers 4 and 5 disposed above the sintering machine through a drum feeder 6 and a cutout chute 7 to form a charged layer 9 also called as a sintering bed. Thereafter, carbonaceous material in a surface part of the charged layer is ignited by an ignition furnace 10 disposed above the charged layer 9, while air above the charged layer is sucked downward through wind boxes 11 located just beneath the pallet 8 to thereby combust the carbonaceous material in the charged layer, and the sintering raw material is melted by combustion heat generated at this time to obtain a sintered cake. The thus obtained sintered cake is then crushed and granulated, and agglomerates of not less than about 5 mm in size are collected as a product sintered ore and supplied into the blast furnace.
In the above manufacturing process, the carbonaceous material in the charged layer ignited by the ignition furnace 10 is continuously combusted by air sucked from the upper part of the charged layer toward the lower part thereof to form a combustion • molten zone having a certain width in a thickness direction (hereinafter referred to as “combustion zone” simply). FIG. 2 is a schematic view illustrating a process wherein the carbonaceous material in the surface part of the charged layer ignited by the ignition furnace is continuously combusted by sucked air to form the combustion zone, which is moved from the upper part of the charged layer to the lower part thereof sequentially to form the sintered cake. Also, FIG. 3(a) is a schematic view illustrating a temperature distribution when the combustion zone is existent in each of the upper part, middle part and lower part of the charged layer within a thick frame shown in FIG. 2.
The strength of the sintered ore is affected by a product of a temperature of not lower than 1200° C. and a time kept at this temperature. In order to manufacture a high-strength sintered ore in a short time and in a high yield with a good productivity, it is required to take some measures for prolonging the time kept at a high temperature of not lower than 1200° C. to increase the cold strength of the sintered ore. It is because a melt starts to be produced at 1200° C. to produce calcium ferrite having the highest strength and a relatively high reducibility among constitutional minerals of the sintered ore in the sintering process. However, the middle part and the lower part in the charged layer are pre-heated by combustion heat of the carbonaceous material in the upper part of the charged layer carried with the sucked air and kept at a high temperature for a long time, whereas the upper part of the charged layer is lacking in the combustion heat due to no preheating and hence combustion melting reaction required for sintering (sintering reaction) is liable to be insufficient. As a result, the yield of the sintered ore in the widthwise section of the charged layer becomes smaller at the upper part of the charged layer as shown in FIG. 3(b). Moreover, both widthwise end portions of the pallet are supercooled due to heat dissipation from the side walls of the pallet or a large amount of air passed, so that the time kept at a high temperature required for sintering cannot be secured sufficiently and the yield is also lowered.
As to these problems, it has hitherto been attempted to increase the amount of the carbonaceous material (coke breeze) added to the sintering raw material. However, it is possible to raise the temperature in the sintered layer and prolong the time kept at not lower than 1200° C. by increasing the addition amount of coke as shown in FIG. 4, while at the same time, the highest achieving temperature in the sintering exceeds 1400° C. and the decrease of the reducibility and cold strength of the sintered ore is caused. When the temperature exceeds the above temperature, calcium ferrite produced at a temperature of not lower than 1200° C. is decomposed into an amorphous silicate (calcium silicate) having the lowest cold strength and reducibility and a skeleton-crystal type secondary hematite easily causing reduction powdering, and hence the high-quality sintered ore cannot be obtained. To this end, it is necessary that the highest achieving temperature in the charged layer during sintering is not made to exceed 1400° C., preferably 1380° C., while the temperature in the charged layer is kept at not lower than 1200° C. (solidus temperature of calcium ferrite) for a long time. In the invention, the time kept in the temperature range of not lower than 1200° C. but not higher than 1400° C. is hereinafter called as “high-temperature zone retention time”.
In order to address the problem, there are hitherto proposed some techniques for the purpose of keeping the upper part of the charged layer at a high temperature for a long time. For example, Patent Document 1 proposes a technique of injecting a gaseous fuel onto the charged layer after the ignition of the charged layer, and Patent Document 2 proposes a technique of adding a flammable gas to air sucked into the charged layer after the ignition of the charged layer, and Patent Document 3 proposes a technique wherein a hood is disposed above the charged layer for attaining a high temperature in the charged layer of the sintering raw material and a mixed gas of air and coke-oven gas is blown from the hood at a position just behind the ignition furnace, and Patent Document 4 proposes a technique of simultaneously blowing a low-melting point flux and a carbonaceous material or flammable gas at a position just behind the ignition furnace.
In these techniques, however, since a gaseous fuel with a high concentration is used and the amount of the carbonaceous material is not decreased in the blowing of the gaseous fuel, the highest achieving temperature in the charged layer during the sintering becomes high exceeding 1400° C. as an upper limit temperature under operation control, so that a sintered ore having a low reducibility or cold strength is formed, and hence the effect of supplying the gaseous fuel is not obtained, or the air permeability is deteriorated due to the temperature rising and thermal expansion by the combustion of the gaseous fuel to decrease the productivity, or further there is a risk of causing a fire accident in the upper space of the sintering bed (charged layer) by the supply of the gaseous fuel. As a result, any of these techniques are not brought into practical use.
As a technique for solving the above problems, the inventors have developed and proposed a technique wherein both of the highest achieving temperature and the high-temperature zone retention time in the charged layer are controlled within adequate ranges by decreasing the amount of the carbonaceous material added in the sintering raw material and introducing various gaseous fuels diluted to not more than the lower limit concentration of combustion into the charged layer from above the pallet in the downstream of the ignition furnace of the sintering machine to perform combustion in the charged layer in Patent Documents 5˜7 and so on.
When the techniques disclosed in Patent Documents 5˜7 are applied to the method of manufacturing the sintered ore and the gaseous fuel diluted to not higher than the lower limit concentration of combustion is introduced into the charged layer while decreasing the amount of the carbonaceous material added to the sintering raw material to combust the gaseous fuel in the charged layer, as shown in FIG. 5, the gaseous fuel is combusted in the charged layer (in the sintering layer) after the combustion of the carbonaceous material, so that the width of the combustion • molten zone can be enlarged in the thickness direction without exceeding the highest achieving temperature over 1400° C. and hence the high-temperature zone retention time can be prolonged effectively.
When sintering operation is conducted by supplying the gaseous fuel, however, it is feared that the gaseous fuel supplied is leaked out of the hood to cause a fire or an explosion in the case of a strong crosswind. It is also feared that the use of the blast furnace gas containing a large amount of CO or the like as the gaseous fuel may lead to a man-made disaster. Thus, the inventors have proposed a hood structure being little in the leakage due to crosswind in Patent Document 8, Patent Document 9 and so on.
FIG. 6 is a view illustrating an outline of a gaseous fuel supply apparatus proposed in Patent Document 9. The gaseous fuel supply apparatus 21 is comprised by arranging a plurality of gaseous fuel supply pipes 23 in parallel at a predetermined interval in the advancing direction of the pallet above a raw material charged layer 9 (sintering bed) and a hood 22 composed of a top-opened vertical wall around the gaseous fuel supply pipes 23. For example, a gaseous fuel such as LNG, town gas or the like is jetted from an injection port of each of the gaseous fuel supply pipes 23 toward a horizontal direction to both sides at a high speed. Above the gaseous fuel supply pipes 23 are arranged baffle plates 24 having a dog-leg shaped cross section in plural rows at intervals in the widthwise direction of the hood and in plural steps at intervals in the vertical direction in zigzag form. It is also disclosed that it is effective to arrange fences 25 having a certain void above the both sides of the hood 22 or provide seal covers 26 between the lower end portions of the both side surfaces of the hood 22 and the side walls of the pallet 8 as a countermeasure for the crosswind.